The present invention relates to a method for treating produced gypsum and, in particular, to a method for treating flue-gas desulfurization gypsum to form highly active, modifiable gypsum binders in high strength.
It is well known that burning facilities of the gypsum industry for the treatment of natural gypsum are used primarily to dehydrate this type of gypsum.
It is repeatedly pointed out in the literature that flue-gas desulfurization gypsum, before it is dehydrated, must be converted into a product similar to natural gypsum. This means that expensive drying and compacting in a roller press are required. In addition, the known methods usually must provide facilities for cooling the dehydrated product.
Since the produced gypsum, dehydrated with the known methods, has processing properties which deviate disadvantageously from those of conventional gypsum binders, such as thixotropy or sedimentation, compensation for these deficiencies is aimed for by an expensive after-treatment, such as an aging or milling treatment or the addition of additives.
An indirectly heated tubular drier at 403.degree. to 453.degree. K. is provided in accordance with DD-WP 263 048 to control the dust and quality problems when dewatering finely-grained gypsum. However, with such a drier, the product is exposed to residence times of about 50 minutes, irrespective of the degree of dehydration of the individual grains.
A fluidized bed drier, in which the fluidized bed consists of sand or of a different grainy material with a good heat transfer coefficient, which is inert to powdered gypsum is disclosed in German patent 3,721,421. By means of this method, gypsum particles are introduced into the fluidized bed so as to be uniformly distributed to absorb the heat from the fluidized bed of sand and from the fluidizing gas.
The fluidized bed of sand, as a means for transferring heat, is technically expensive compared to methods in which the fluidized bed at the same time is the fluidized material. A different fluidized bed method disclosed in German Auslegeschrift 2,622,994 provides a fluidized bed above a whirling bottom with air preheated to about 378.degree. K. and a heat exchanger, which makes possible the dehydration of the produced gypsum in contact with the fluidized bed. It is a disadvantage of this method that the produced gypsum must be dried to a powder, before it is added. The material, heated to 445.degree. to 465.degree. K., must subsequently be cooled again over heat exchangers in the fluidized bed. This is technically expensive.
Various fluidized beds apparatuses and methods or for drying pastes and finely grained products are known, for example, from U.S. Pat. No. 4,581,830. References to complex process steps, that is, to drying, deagglomerating and dehydrating or calcining, generally are lacking. Modern, industrially-offered equipment or installations also effectuate the process steps of drying, deagglomerating and dehydrating or calcining with different apparatuses of different geometry.
It is an outstanding characteristic of all these known methods that, to attain the required degree of dehydration, material temperatures must be employed which lie in the range of 393.degree. to 473.degree. K. Aside from the dust precipitation equipment, several pieces of equipment are always used. A plurality of apparatuses and high temperatures are disadvantageous from an energy point of view.
German patent 2,727,544 discloses fluidized bed methods for drying and burning gypsum dihydrate to gypsum hemihydrate. These methods fulfill the task in one apparatus, but require conversion temperatures of 393.degree. to 433.degree. C.